Some images of the Sword Coast show the Lizard Marshes south of the mouth of the Delimbiyr, and others show it ending at the Lizard Marshes.
The description from Forgotten Realms Campaign Setting 2nd edition (revised) and Storm King's Thunder says
Eventually, the river dissolved into a morass of waterways, forming a vast swamp known as the Lizard Marsh.
Further, according to Scourge of the Sword Coast,
The waterways of the Delimbiyr at Daggerford were too shallow for ships to continue upriver into the rest of the Delimbiyr Vale.
This seemingly contradicts Storm King's Thunder
In this stretch, the Delimbiyr was especially wide, giving the settlement enough room for a modest harbor.
Unless that's meant to imply the Delimbiyr is navigable northeast of Loudwater. But then why would ships be at Daggerford in the first place, if the mouth is not traversable and neither is anything south of Loudwater?
Maybe it's navigable from the Sword Sea to Daggerford, and then from Loudwater northeast, but not between Daggerford and Loudwater?